Different concepts are adopted as the drive system of roller mills, for example DE 20 2009 016 825 U1 describes, for example, a roller mill having a grinding plate and a plurality of grinding rollers which roll on the grinding plate, wherein only the grinding plate are driven by means of at least two drives with a motor and gear mechanism. The at least two drives engage here in a common gear rim, with the result that all the drives have to rotate with the same rotational speed. In order to homogenize the load, at least one of the drives is provided with a turbocoupling.
DE 10 2006 050 205 A1 also discloses a roller mill whose grinding plate is driven by an arrangement of more than two drives. Electric motors which are fed via frequency convertors and which are used to regulate the rotational speed and torque are provided for the drives. The frequency convertors are organized according to the master-slave principle in order to ensure that all the drives operate synchronously. However, these frequency convertors result in high costs for the drive trains.
Furthermore, it is known, for example, from DE 10 2008 036 784 A1 to drive the grinding rollers instead of the grinding plate. In order to reduce the costs for the regulating devices, this application proposes that the grinding rollers each have a motor with a rotor winding and at least one regulating device is provided for regulating the motor torque of at least one drive, wherein the regulating device is connected here to the rotor winding of at least one drive in order to influence the rotor current. The influencing of the rotor current can take place, for example, by means of convertors whose power is adjusted, with this method of applying influence, according to the rotational speed deviation between the operating point and the rated point, generally ≦40% of the rated motor power. It is therefore possible for convertors with a substantially lower power to be used, and since the convertor costs are virtually proportional to their power it is possible to achieve a significant cost saving here.
WO 2009/030609 A1 describes a method for comminuting material to be ground with a roller mill in which at least two grinding rollers are provided with assigned drives, wherein a power compensation regulation is carried out for the two drives in that by regulating the grinding pressure of at least one of the grinding rollers the power of the drives are regulated in a predefined ratio with respect to one another.
DE 10 2007 033 256 A1 discloses a roller mill which comprises a main drive and an additional drive for driving the grinding plate, wherein a regulating device regulates the additional drive as a function of torque fluctuations of the main drive and/or fluctuations in the rotational speed of the grinding plate.